1. Field of the Invention
The present invention relates to a method for production of glass bodies by a redrawing method. The glass bodies consist of photo-structurable glass.
2. Description of Related Art
Re-drawing methods are known to the skilled person from the prior art. They are suitable for producing a thin glass sheet, which is usable in different applications, from a glass body (blank).
For example U.S. Pat. No. 3,635,687 discloses a method, in which a glass body is drawn to a thin glass via a redrawing method. In this document a soda-lime glass is used for the glass body. Soda-lime glass is a widely used technical glass, which has very good chemical resistance and low crystallization tendency due to its high SiO2-content. Therefore this glass can be handled well in a redrawing method.
During redrawing of a glass, a blank is partially heated until the viscosity in the heated region is sufficient for drawing the blank to increasing length. The thickness of the glass decreases and the length increases due to the drawing. Usually also the width of the glass body decreases. U.S. Pat. No. 7,231,786 B2 tries to counteract this decrease of width by different apparatus-based measures.
During redrawing a glass body is heated up to a temperature at which it can be drawn to increasing length. This is a process that may result in crystal formation in crystallization-sensitive glass. Thereby the glass becomes unusable. Consequently, redrawing methods have been applied so far only to technical glasses, which are not particularly crystallization-sensitive, and to particularly crystallization-resistant optical glasses.
It has to be differentiated between unwanted crystallization in glass production as described above, which is also called “devitrification”, and directed crystallization, which can be achieved in photo-structurable glasses by light exposure and tempering. It is indeed a crystallization process in each case, however, the processes differ with regard to the occurring crystals. While alkali disilicates are formed in devitrification, alkali metasilicate crystals are generated in directed crystallization in photo-structurable glasses. Despite the described differences with regard to the processes underlying crystallization, photo-structurable glasses also have an increased tendency to devitrification in addition to the tendency to crystal formation induced by exposure to light and tempering desired for photo-structurability.
All prior art methods for redrawing have in common that they serve for production of glass bodies for use as display glasses or other technical flat glasses. It has been partially worked out to process optical glasses in flat glass methods as for example described in DE 10 2010 042 945 A1. However, special care had been taken that the glasses are crystallization-stable in order to enable processing in such methods.
Photo-structurable glasses are generally characterized by a pronounced crystallization tendency, which is desired for structuring. For example, these glasses contain nucleating agents, which form metal nuclei upon directed UV-irradiation of defined areas of a glass body. The following reaction equation exemplarily illustrates this event for silver as nucleating agent.Ce3++Ag++hv→Ce4++Ag0  (Equation 1)
Upon irradiation of photo-sensitive glasses with UV-light at a wavelength of about 320 nm, trivalent cerium yields an electron to the silver ion, whereby elemental silver is generated. In a subsequent tempering step the desired crystal nuclei are formed around the generated atomic silver.
It is a challenge for producers of photo-sensitive glass to adjust a good balance between the desired crystallizability after production and the critical crystallization tendency, which complicates the production. For example, increase of the applied amount of silver results in formation of elemental silver already during melting. This precipitates and may result in silver bubbles or silver droplets, whereby the production is complicated or even becomes impossible.
Furthermore, production of photo-structurable glass is also particularly challenging for the reason that the components cerium and silver, respectively, have to be present in the correct oxidation state in the final glass so that the reaction as described above (Equation 1) can take place. Of course, an oxidizing melting procedure results in prevention of precipitation of silver during melting because reduction to metallic silver does not occur. However, in such a case, cerium is present in its tetravalent form in the glass so that the desired reaction (Equation 1) cannot take place upon exposure to UV-light.
On the other hand, if a reducing melting procedure is selected, for increasing the amount of trivalent cerium, the risk is increased that elemental silver is formed already during production. Silver nuclei in unexposed glass disturb selective crystallization of the glass by tempering because also unexposed areas would crystallize. Moreover, the glass would not fulfill the transmittance requirements. Rather, it is desired that no elemental silver is formed during production so that essentially the entire silver in the glass is available as monovalent silver ions for the reaction of Equation 1.
In addition to the pair of nucleating agents, which generally represents only a very small proportion of the glass, the remaining composition of the glass is of course also contributing to crystallization sensitivity. The attention of the developers of such glasses has so far been more directed to increasing the crystallization tendency onto a level necessary for subsequent structuring. Therefore, manufacturing of photo-structurable glasses in redrawing methods was not considered so far. Such glasses would be crystallizing during heating.